Guinea pig myelin basic protein will be prepared and treated with pepsin to yield 3 major fragments. One of these, residues 44-89 (P1) contains the full encephalitogenic activity of the parent molecule in the Lewis rat. A series of overlapping peptides will be prepared from the P1 fragment and each peptide examined for the ability to induce experimental allergic encephalomyelitis, cell-mediated immunity, or humoral antibody. In addition, each peptide will be tested for reaction with preformed anti-P1 antibody. These studies will define the amino acid sequences necessary for the above immunological activities. In addition, chemical modification studies will be done which will define critical amino acid residues within the sequences required for biological activity.